It's quite versatile with it's sounds which can range from sparkling clean to a heavy, sustain filled Overdrive. The cleans break up nicely with high gain (and powerful pickups). It's not noisy but it does physically give off a 'hum' (probably the power supply) but it can't be heard through any of the outputs so I don't mind.
For live use I hook it up through a Compressor/Gate > 31-band EQ > Stereo Digital Effects (for reverb etc) and then into a Marshall 20/20 with a 1922 Cab. For band practice it goes through a Marshall MG30 combo and at home I use the JMP on it's own with headphones.
The speaker emulated outputs are great for recording. The 'Bass Shift' vibrates through your chest when combined with the 'Deep' switch on the 20/20 :-)
Haven't had any problems with it but the volume settings (not master volume) need to be lowered to eliminate some annoying clipping (even with just the headphones!). I'm not sure if it's just on high output pickups but even the factory defaults needed to be lowered. It also looses volume sometimes within the first few minutes of being powered on but the 20/20 takes a few minutes to warm up too so I don't mind.
I got this one second hand with some spare Marshall Valves thrown in. I don't care if it's All-Valve or not, It sounds great! I'm planning to get another one sometime to keep permanently in the rack as removing it after live shows will probably start annoying me.